Various kinds of material are fed to incineration furnaces, for the most part fossil fuels, that is to say fuels which are tens of thousands of years old and older. In particular in the case of waste incinerators, but also with other incineration furnaces, a fuel stream may also be supplied which comprises fuels which are less old. Examples thereof are wood, rubber material and the like. These fuels are also referred to as biogenic (or biogen) fuels (“biomass”).
For environmental reasons, it is desirable to limit the emission of the greenhouse gas CO2 as much as possible, in particular the CO2 which is generated during the combustion of fossil fuels. Combustion of carbon which has been converted to, for example, wood relatively recently is not considered to be polluting. After all, wood is only a few tens to hundreds of years old and is the result of the conversion of carbon dioxide from the atmosphere to carbon.
In certain countries, levies have even been introduced for the emission of fossil fuel, that is to say fuel which originated more than 10,000 years ago.
If a mixed fuel stream is fed to an incineration furnace, it is important to know which fraction of the supplied fuel is fossil and which fraction of the fuel is more recent.
Fossil and recent (non-fossil) carbon can be distinguished by their 14C content. Non-fossil carbon originates from the atmosphere relatively recently, and atmospheric carbon is enriched in 14C by cosmic irradiation of 14N. As a result, atmospheric carbon contains approximately 1 ppt (1 per 1012) of 14C on total carbon.
14C is a radioactive element having a half-life of approximately 5700 years, which means that within said period, half of the 14C content of any organic material which does not take part in the carbon cycle will have decayed (back to 14N). Consequently, essentially all 14C will have disappeared within about 60,000 years if the carbon was stored underground. In other words, the 14C content of fossil fuels is zero, while in recent biogenic fuels it is essentially the same as it is in the atmospheric carbon (approximately 1 ppt).
The CO2 which is present in a waste gas may partly originate from fossil fuels and partly from biogenic fuels (biomass). By determining the 14C content, the amount of fossil and biogenic fuel, respectively, can be known. If the total CO2 percentage is known, the amount of fossil fuel in a sample can be calculated in a simple manner. Using the results, it is possible to determine, for example, charges and the like.
One possibility of determining this is through the analysis of the stream which is supplied. Such a method is inaccurate when only random checks are carried out. In addition, it requires highly experienced investigators and/or is complicated.
WO2002/06730 describes a method for determining the relationship between fossil and non-fossil energy carriers in a fuel mixture by determining the 14C/12C and/or 3H/1.
WO2009/054718 describes a method and device for determining the fossil fuel content in a fuel stream fed to an incineration furnace. The waste gas is sampled in a sampling container, presumably in gas form, and the amount of “new” and “old” carbon present in the CO2 is determined by analysing the level of 14C of the CO2 in the sampling container, e.g. using liquid scintillation counting. The handling of the sampled CO2 in gaseous form, however, is cumbersome, especially where the analysis site is at a distance from the sampling site
DE 10159967 discloses a method of collecting 14C from off-gas of nuclear power plants, by using caustic soda which is absorbed on a supporting web. The amount of absorbed CO2 from the off-gas is used as a measure of the amount of off-gas which has been sampled. and then assaying the level of 14C in the collected CO2.
A disadvantage of prior art is that on-line analysis of 14C is not possible. This implies a storage of the CO2. It would be possible to store CO2 in a liquid, such as a NaOH solution. A disadvantage however of liquid storage mediums is that they are more difficult to transport and are less robust. Further, the reliability of liquid storage may be less than desired.